Support for PyPy3

[Jason Carver]

This is a tracking issue for PyPy3 support.

Currently, the missing component is PyUnicode_New. I posted that issue to the pypy project:

https://bitbucket.org/pypy/pypy/issues/2723/undefined-symbol-pyunicode_new-in-pypy3

[Christian Heimes]

Thanks, the function is the preferred way to create unicode objects, from https://docs.python.org/3/c-api/unicode.html?highlight=pyunicode_new#c.PyUnicode_New

This is the recommended way to allocate a new Unicode object. Objects created using this function are not resizable.
New in version 3.3.

[Christian Heimes]

pysha3 is no longer supported. Please use SHA-3 from Python's hashlib module.